IPv4 means Internet Protocol version 4, whereas IPv6 means Internet Protocol version 6.It uses a 32 bit addressing and allows for 4,294,967,296 unique addresses.Ipv4 has four different class types, the class types are A, B, C, and D.The IPv6 header has a new header format that is designed to minimize header overhead.This optimization is achieved by moving both non-essential fields and optional fields to extension headers that appear after the IPv6 header.

IPv6 is based on IPv4, it is an evolution of IPv4. So many things that we find with IPv6 are familiar to us. The main differences are:

1.Simplified header format. IPv6 has a fixed length header, which does not include most of the options an IPv4 header can include. Even though the IPv6 header contains two 128 bit addresses (source and destination IP address) the whole header has a fixed length of 40 bytes only. This allows for faster processing.

Options are dealt with in extension headers, which are only inserted after the IPv6 header if needed. So for instance if a packet needs to be fragmented, the fragmentation header is inserted after the IPv6 header. The basic set of extension headers is defined in RFC 2460.

2.Address extended to 128 bits. This allows for hierarchical structure of the address space and provides enough addresses for almost every 'grain of sand' on the earth. Important for security and new services/devices that will need multiple IP addresses and/or permanent
connectivity.

3.A lot of the new IPv6 functionality is built into ICMPv6 such as Neighbor Discovery, Autoconfiguration, Multicast Listener Discovery, Path MTU Discovery.
4.Enhanced Security and QoS Features.

Answer:
IPv4 means Internet Protocol version 4, whereas IPv6 means Internet Protocol version 6.
IPv4 is 32 bits IP address that we use commonly, it can be 192.168.8.1, 10.3.4.5 or other 32 bits IP addresses. IPv4 can support up to 232 addresses, however the 32 bits IPv4 addresses are finishing to be used in near future, so IPv6 is developed as a replacement.
IPv6 is 128 bits, can support up to 2128 addresses to fulfill future needs with better security and network related features. Here are some examples of IPv6 address:

1050:0:0:0:5:600:300c:326b
ff06::c3
0:0:0:0:0:0:192.1.56.10

The most important difference is that it has a larger address space. IPv6 uses 128 bits, instead of the 32 bits used in an IPv4 address.

There are also some changes in the header format, and some additional options, like built-in security options. These can be added to IPv4 through additional protocols, so this is really no big deal.